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Keto structure

XVIII has now been excluded,42 since the methyl glycoside of the sugar reacts rather rapidly with one mole of periodate per mole furthermore, in contrast to the behavior of mycaminose (XXI), no moiety with one carbon atom less has been isolated from the products of periodate oxidation. Oxidation of mycarose with hypoiodite affords a crystalline lactone with the empirical formula C7H12O4 this observation eliminates the possibility of the keto structure XX. Thus, mycarose appears to be a 2,6-dideoxy-3-C -methylhexose (XIX). In the original compound, the isovaleryl group must be esterified to the alcohol function at C4, since the methyl glycoside isovalerate obtained from carbomycin is only attacked by periodate after alkaline hydrolysis. [Pg.273]

This enediol can be regarded as a common enol tautomer for two different keto structures. In other words, there are two ways in which this enediol can tautomerize back to a keto form, and the reaction thus appears to shift the position of the carbonyl group. The reaction is enzyme catalysed, which allows the normal equilibrium processes to be disturbed. [Pg.351]

H may return to C , to give the more stable carbonyl compound (the keto structure), or to O, to give the less stable enol. [Pg.386]

With few exceptions (Problem 17.6) the keto structure rather than the enol is isolated from reactions (see Section 8.2, item 4). [Pg.386]

Radical Cations with Biological Relevance. - The radical cation of 1-ben-zyl-l,4-dihydronicotinamide, an analogue of NADH, was generated by oxidation with Fe(bpy)33+ and Ru(bpy)33+ (bpy = 2,2 -bipyridine) in deaerated acetonitrile solutions using a rapid-mixing flow apparatus. The ESR spectra reveal a keto structure of the radical cation, rationalized by DFT calculations. Upon photo-induced electron transfer, also the enol form could be established, which, however, relaxes to the keto form131,132. [Pg.94]

Fig. 10 The three-dimensional potential energy surface describing the motion of protons between N6(A) and 04(T) and between N3(T) and N1(A) shows two critical points in the ground state. The deeper minimum corresponds to the amine/keto structure of AT and a shallow one to the imine/enol structure (A T ). Upon absorption of a UV photon the initaly delocalized excitonic states (1) undergo a rapid localization on f 10 ps timescale for single bases and 100 ps timescale for stacked base pairs to form a charge transfer (CT) states. The subsequent CT states passing through a conical intersection are carried back to the ground state. Fig. 10 The three-dimensional potential energy surface describing the motion of protons between N6(A) and 04(T) and between N3(T) and N1(A) shows two critical points in the ground state. The deeper minimum corresponds to the amine/keto structure of AT and a shallow one to the imine/enol structure (A T ). Upon absorption of a UV photon the initaly delocalized excitonic states (1) undergo a rapid localization on f 10 ps timescale for single bases and 100 ps timescale for stacked base pairs to form a charge transfer (CT) states. The subsequent CT states passing through a conical intersection are carried back to the ground state.
During an alkaline oxygen stage the formation of keto structures by oxidation of ketols is postulated, finally leading to acids. Sodium permanganate treatments yield a moderate increase in both, carbonyls and carboxyl groups at short reaction times [20]. [Pg.6]

Scheme 4 Cleavage of the glycosidic bond by -elimination starting from C2-keto structures along the cellulose chain... Scheme 4 Cleavage of the glycosidic bond by -elimination starting from C2-keto structures along the cellulose chain...
Although methyl 4-oxotetrahydrothiopyran-3-carboxylate and the corresponding 2,3-dihydro-4//-thiopyran-4-one exist as a mixture of keto and enol forms in solution, the 2,3-dihydro sulfoxide is present only as the ketone. However, the sulfone and the tetrahydro sulfoxide and sulfone are present exclusively in the enolic form. The picture is further complicated in the solid state, where the dihydro compound and its sulfoxide exist exclusively as the keto structure but the sulfone is found as the enol (Table 27) <1986J(P2)1887>. The related 2-esters of tetrahydrothiopyran-3-one 268 exist as a ca. 1 2 mixture of keto and enol <1981T2633>. [Pg.805]

In imidazotropone 105a, the keto structure is supported by its spectral similarity to nontautomeric A-benzylated derivatives (69MI1). Even in pyr-rolotropone 181, whose carbonyl group is in a /3 position relative to the bridgehead, the keto form is supposed to dominate (65BCJ306). [Pg.310]

Even so, you will see in Chapter 22 that intermediates with this keto structure are formed in reactions on the benzene ring of phenols. Like ascorbic acid, phenol is also quite acidic (pfCa 10) and used to be called carbolic acid. [Pg.533]

The formation of these compounds has already been dealt with (Section III, A). No firm evidence is available to indicate whether oxo (183) or hydroxy (184) structures are favored in this series. In the case of 4-substituted isoxazolin-5-ones, N—H keto structures usually predominate,58 324 and it would be surprising if they did not... [Pg.334]

A structure with —OH attached to doubly-bonded carbon is called an enol -ene for the carbon carbon double bond, -ol for alcohol). It is almost always true that when we try to make a compound with the enol structure, we obtain instead a compound with the keto structure (one that contains a C= 0 group). [Pg.261]

Some unusual rearranged tricyclic diterpenoids with a cis- a/b ring fusion have been isolated62 from Rondeletia panamensis (Rubiaceae). Panamensin has the structure (40) whereas its dihydro-derivative, rondeletin, has the 2-keto structure (41). [Pg.193]

The relative stabilities of the tautomers of dicoumarol, which can potentially exist in six tautomeric forms, and its derivatives were studied by the PM3 method (04JST(678)55). The calculations have been carried out both in the gas phase and in a continuum model of water, on isolated tautomers of 10 systems obtained by single proton transfer. In the most cases, the predominance of the a,/-benzopyran tautomeric structure 154a was predicted in the gas phase. In aqueous solution, the calculations imply significant qualitative differences in the relative stabilities of the tautomers and predict the predominance of the tetra-keto structure 154b, even though the a,a -benzopyran structure is the one with the largest dipole moment. [Pg.65]

For most carbonyl compounds, the keto structure is greatly preferred, mainly due to the extra strength of the C = 0 bond. However, the enol form is stabilised if the C=C bond is conjugated with a second Jl-system or if the OH group is involved in intramolecular hydrogen bonding. Example... [Pg.135]

Physical methods have been invoked more and more in order to obtain information concerning the keto structure and to determine properties characteristic of this class of sugar. Ultraviolet and infrared spectroscopy have been used for identification of the free carbonyl group, and the latter technique also permits the identification of all 2-ketoses by the... [Pg.236]


See other pages where Keto structure is mentioned: [Pg.175]    [Pg.688]    [Pg.894]    [Pg.426]    [Pg.451]    [Pg.171]    [Pg.416]    [Pg.416]    [Pg.137]    [Pg.20]    [Pg.166]    [Pg.435]    [Pg.172]    [Pg.285]    [Pg.172]    [Pg.84]    [Pg.416]    [Pg.261]    [Pg.324]    [Pg.177]    [Pg.96]    [Pg.275]    [Pg.40]   
See also in sourсe #XX -- [ Pg.252 ]




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